A focus sensing system is an optical device that senses the state of focus of an image-forming device by emitting one or more beams of light that are made to reflect from the surface under examination. The reflected light is then collected and processed to determine the distance between the objective lens and the target object being examined. Such a focus sensing system may simply measure the distance between the objective lens and the target, or it may be used to enable control of the state of focus. In the latter instance, it may be used as a component of a focus controlling system, that is, an autofocus system.
Many devices in the prior art project a spot of light through a circular objective lens onto a target surface. Other focus sensing devices have used one or more toric lenses to form a line focus which is projected onto a target surface. In general, a toric lens is one which has different curvatures in two principle sections. A cylinder lens is a limiting case of a toric lens where the radius of curvature in one section is infinite, that is, the section is flat, while the radius of curvature in the other section is finite, but non-zero. Toric lenses introduce astigmatism into the outgoing beam of light. Instead of having a single point focus, an astigmatic system has two line foci. Where astigmatic lenses have been used previously by others, only one of the two astigmatic line foci has been used. In those applications the line focus has been used to perform an averaging function in order to reduce the effect of local asperities in the surface under observation.
While such an arrangement can be optimized for measuring the distance to a single reflective surface, it has fundamental difficulties when it is used to measure distance to an object or surface that is buried in a transparent medium. These problems result from the fact that the discontinuity in the refractive index at boundaries, even between transparent interfaces, still produces a reflection. Such a reflection creates its own signal, confusing the returned position signal from the intended target. The result is degraded image quality because of imperfect focus.
Various techniques have been used in attempts to overcome these error mechanisms. For example, an electrical, mechanical or optical offset may be introduced to offset the signal from an unwanted surface. Unfortunately this offset can only correct the net focus error signal if the reflectance of the unwanted surface does not change, or the distance between the surfaces remains constant. The general result is that previously used methods remain susceptible to many factors, resulting in errors in the distance measurement and a consequential degradation in the quality of the image.